fix(http): expose Agent keep-alive timeout buffer - #8608

Summary

• retain Node-compatible agentKeepAliveTimeoutBuffer constructor values and default invalid or missing values to 1000 ms
• expose the ordinary writable Agent property through Perry's external-handle get/set dispatch, including GC rooting for assigned JS values
• add a parity regression mirroring Node's test-http-agent-keep-alive-timeout-buffer.js assertions

Tests

• cargo test --release -p perry-ext-http agent::tests -- --nocapture (8 passed)
• cargo fmt --all -- --check
• git diff --check
• python3 scripts/check_test_registration.py
• Node output matches the new parity fixture exactly

Refs #4975

No version bump.

Summary by CodeRabbit

• New Features

  • Added support for the agentKeepAliveTimeoutBuffer option on HTTP and HTTPS agents.
  • Valid finite, non-negative values are preserved; invalid or missing values default to 1000 ms.
  • The setting can be read and updated after an agent is created.

• Tests

  • Added coverage for valid, invalid, infinite, and omitted timeout buffer values.